The present invention relates to a method and a device for starting single-phase asynchronous motors particularly for motors for low-power actuation systems.
As is known, when a three-phase line is not available, it is possible to use single-phase asynchronous motors, also known as induction motors, which are constructively similar to three-phase asynchronous motors but have only two stator windings, which are phase shifted by 90° and through which currents phase shifted by 90° flow: a run winding and a start winding. The presence of the start winding is necessary to start induction motors, which otherwise would not be able to start spontaneously.
The phase shift between the currents that flow through the two windings, since they are both powered by the same voltage, can be achieved by using a resistive start winding, or by connecting in series to the start winding a start capacitor of appropriate capacitance. Whereas in the second solution the start capacitor can remain permanently inserted or can be disengaged once starting has occurred, if a resistive start winding is used, it is necessary to disengage it after starting.
The simplest solutions for disengaging the start windings are constituted by centrifugal switches, which open or close the circuit as a function of the rotation rate, or by maximum-current or maximum-voltage relays, which open or close the circuit respectively as a function of the current intensity that flows through their coil or of the voltage to which their coil is subjected.
That type of device, being composed of mechanical parts and electrical parts, has many problems linked in particular to the wear that affects both the mechanisms and the electrical contacts.
In order to obviate those problems, electronic starters have been devised which are substantially constituted by a thyristor, connected in series to the start winding, by a pair of bimetallic contacts and by a sensor coil, which is arranged inside the motor. When the motor is connected to voltage for starting, the bimetallic contacts that drive the control input of the thyristor are closed, making it conduct and consequently closing the circuit of the start winding. As the rotation rate increases, the voltage of the sensor coil, which is proportional to the rotation rate of the motor, increases, thus generating an amount of heat that is sufficient to deform the bimetallic contacts, which open and shut down the thyristor. In this manner, the flow of current is interrupted in the start winding and the motor continues to rotate only by virtue of the run winding.
Although those electronic starters perform their task well, they have some limitations when they are subjected to challenging operating conditions and in particular when they are subjected to repeated starts which are very close in time. When the motor stops, it is in fact not immediately ready for a new start, since it necessarily requires an idle time of a few seconds after interruption of the power supply in order to allow the bimetallic contacts to cool down sufficiently to straighten again and be able to make the thyristor conduct, in order to close again the start circuit. Although this drawback may seem scarcely important, in practice it can become very significant depending on the application that the induction motor controlled by the electronic starter is required to perform. One should consider, for example, the case in which these motors are associated with pumps which must supply as regularly as possible a given user device, or that must keep as constant as possible the level of a container filled with water and therefore are subjected to repeated starts that are close in time.
A further aspect relates to the arbitrariness of the time needed to restore the initial conditions in this type of electronic starter after the motor has stopped. In fact, this restore time corresponds substantially to the time that is physically necessary to allow the bimetallic contacts to cool down and close, straightening themselves; therefore, in addition to being difficult to reproduce identically on each different electronic starter, in practice it is not modifiable, except exclusively during design and manufacture.